In the current generation of radio frequency MEMS switches there is need for improvement in the areas of power handling, temperature stability, switching speed, radio frequency switch capacitance ratio, and integration-friendly packaging. The invention described here offers the potential for improved performance in these areas. Although the improvements to be described here are especially relevant to the capacitance coupled radio frequency switch, readers skilled in the MEMS switch art will recognize relevance of several included concepts to the ohmic contact or direct current MEMS switch. The improvements considered here also involve switch processing—including single wafer MEMS device fabrication and consideration of physical protection for a new, thin and fragile, easily destroyed, switch element.
In a conventional spring-force restored MEMS switch, the ability to hot-switch radio frequency (RF) power is often limited by radio frequency (RF) latching of the device. RF latching occurs when the holding force created by applied RF signal voltage exceeds the restoring spring-force of the switch beam or bridge or movable arm member. For typical capacitive switches, RF hot-switching power is realistically limited to less than about 1 watt. Even at such moderate power, the lifetime of a switch will often be reduced because the presence of the RF voltage reduces the amount of dielectric charging the switch can sustain before failure. Although many system applications do not require hot-switching, a switch that is capable of hot switching can simplify the design of a system using the switch.
Operation over wide temperature ranges is another concern for RF MEMS switches. In devices using a metal bridge, a significant fraction of the spring constant typically arises from
Tension in the beam. If the thermal expansion coefficients of the beam and substrate are not well matched, the spring constant will vary significantly with temperature. This temperature-induced variation in spring constant reduces the safe operating margins of the device because the device must be stiff enough to operate at high temperature, but not so stiff that the operating voltage is excessive at low temperature.
Switching speed of the RF MEMS switch is determined by the net actuator force, the mass of the moving structure, the distance the structure moves, and the damping of the atmosphere surrounding the moving structure. In the zip-mode or touch-mode or S-shaped switch arm MEMS switch of special interest in the present invention, the net actuator force can be further increased by removing the opposing spring force of the moving beam. Optimization of the switching speed can be completed by making the moving beam less massive, and by removing the damping atmosphere surrounding the beam. Unfortunately however, for most spring-force restored devices, their relatively high mechanical Q-factors result in many milliseconds of ringing upon opening when operating in even moderate vacuum.
For parallel-plate actuated spring-force restored RF MEMS devices, there is a critical trade-off between operating voltage, restoring force, and capacitance ratio. For devices operating at the same voltage, the restoring force of the device can be increased by decreasing the open-state gap and increasing the beam spring constant. As a result, the increased restoring force required for reliable operation is achieved by reducing the on-state to off-state capacitance ratio of the device.
Finally, the packaging scheme for an RF MEMS switch should be compatible with a monolithic microwave integrated circuit practices for applications such as phase shifters, switchable filters, and signal routing networks. Ideally, the packaging approach used should have low RF losses, should be implemented using standard fabrication processes, and should protect the switching component elements from the environment before the device leaves the clean fabrication area. A thin-film packaging approach as described herein meets all of these goals. In addition, a device that can operate in a low pressure environment allows sealing of the thin-film package by a wider range of vacuum deposition techniques than would be available for devices requiring gas damping.
With respect to the current state of the MEMS switch art it is notable that non-RF devices using S-shaped actuators have been previously demonstrated by Shikida et al. as is disclosed in the reference paper identified in the disclosure filed with the present patent document. Additionally a Gold-contact RF switch using an S-shaped actuator has been disclosed previously by Oberhammer et al. as is similarly disclosed; this device achieves an insertion loss of 2.8 dB and an isolation of 30 dB at 15 GHz. In both of these instances however, the S-shaped actuator is formed through use of a double a wafer-bonding process rather than on a single wafer and the benefits of new self-latching switch operating mode disclosed herein appear to have been unrecognized.